Methods for checking the plausibility of a cylinder pressure sensor

ABSTRACT

A method for determining an amount of energy released in the working cycle of an internal combustion engine cylinder includes: (a) recording a time curve of the rotational speed of the engine crankshaft using tooth timings measured using a toothed sensor disc, (b) assigning each tooth timing to a working cycle of a selected cylinder, (c) determining a cylinder-specific average value from the tooth timings assigned to the selected cylinder, (d) determining cylinder-specific tooth timing deviations from the determined cylinder-specific average value, for the tooth timings assigned to each working cycle of the selected cylinder, (e) determining a cylinder-specific characteristic tooth timing by summing the determined tooth timing deviations, and (f) specifying the amount of energy released in the working cycle of the selected cylinder as a function of the determined cylinder-specific characteristic tooth timing, the amount of energy released being indirectly proportional to the determined cylinder-specific characteristic tooth timing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/409,602 filed Dec. 19, 2014, which is a U.S. National StageApplication of International Application No. PCT/EP2013/062296 filedJun. 13, 2013, which designates the United States of America, and claimspriority to DE Application No. 10 2012 210 301.5 filed Jun. 19, 2012,the contents of which are hereby incorporated by reference in theirentirety.

TECHNICAL FIELD

The present invention generally relates to the technical field of theoperation of internal combustion engines in a motor vehicle. The presentinvention especially relates to (a) a method for determining the amountof energy released in the working stroke of a cylinder of an internalcombustion engine. The present invention further relates to (b) a methodfor the regulation of the smooth running of an internal combustionengine, (c) a method for determining the cylinder pressure in differentcylinders of an internal combustion engine with at least two cylinders,wherein one cylinder is a lead cylinder fitted with a cylinder pressuresensor and the at least one other cylinder is an auxiliary cylinder, and(d) a method for checking the plausibility of a measurement signal of acylinder pressure sensor of an internal combustion engine comprising atleast two cylinders, each fitted with a cylinder pressure sensor.Moreover, the present invention relates to (e) a device for determiningthe amount of energy released in the working stroke of a cylinder of aninternal combustion engine, (f) an engine controller with such a deviceand (g) a computer program that is configured to carry out one of theabove-mentioned methods.

BACKGROUND

In internal combustion engine the masses of fuel that are injected perworking cycle into the individual cylinders vary significantly becauseof manufacturing tolerances of a fuel injection system and by theoccurrence of ageing of components of the fuel injection system.However, differences in the masses of the injected fuel result in torquedifferences between the individual cylinders that have an adverse effecton the smooth running of the internal combustion engine. Modern internalcombustion engines, especially diesel engines, are therefore fitted withat least one so-called cylinder pressure sensor, which detects the timeprofile of the pressure in the interior of a cylinder. The torqueprovided by the cylinder involved can be estimated from the pressureprofile and especially from the level of the pressure during theso-called working stroke in which the fuel combustion takes place. Basedon a knowledge of such torque differences, balancing of the cylinders,i.e. equal torque contributions by all cylinders, can be achieved bymeans of an adjusted cylinder-specific fuel injection.

However, the output signal of a cylinder pressure sensor can beincorrect for many reasons. If such errors are not detected, thistypically results in an incorrect cylinder-specific adjustment of thefuel injection. The smooth running of the internal combustion engine maynot only not be improved but may even be significantly worsened.

A method for so-called cylinder balancing in relation to the injectedmasses of injected fuel in the different cylinders of an internalcombustion engine is known From DE 197 20 009 A1. With this method therevolution rate or the rate of rotation during expansion and therevolution rate or the rate of rotation during compression is calculatedfor each cylinder. The difference in revolution rate between expansionand compression is filtered by means of a smoothing average valuegeneration. Based on said filtered difference in revolution rates, anindividual correction for the mass of fuel is calculated for eachindividual cylinder and said individual correction is taken into accountduring the calculation of the entire mass of fuel to be injected. Thesmooth running of the internal combustion engine can thus be improved bymeans of a mathematically relatively complex algorithm.

A method for compensating a systematic error in injection processes foran internal combustion engine is known from DE 197 00 711 A1. With thismethod, a correction value for the injection timing is used depending onthe rough running.

A method and a system for cylinder balancing in reciprocating pistonengines by compensating the harmonic components of the revolution rateof the crankshaft are known from DE 10 2005 047 829 B3. With this methoda time interval of at least one revolution of the camshaft or tworevolutions of the crankshaft is considered and within said time windowa revolution rate signal of the crankshaft is subjected to a Fourieranalysis.

The most frequent and fundamental cause of rough running of an internalcombustion engine is, however, as explained above, a variation of theinjected masses of fuel in the different cylinders. Assuming completefuel combustion, different fuel-injected masses nevertheless result indifferent amounts of energy being released by fuel combustion in theworking stroke of a cylinder of a four-stroke internal combustionengine.

SUMMARY

One embodiment provides a method for determining the amount of energyreleased in the working stroke of a cylinder of an internal combustionengine. The method includes the recording of a time profile of therevolution rate of the crankshaft of the internal combustion engineusing toothing times, each representing a period of time within whichtwo adjacent teeth of a sensor disk, which is connected to thecrankshaft and which comprises an alternating arrangement of teeth andtooth spaces along its circumference, pass a reference position;associating the toothing times with a respective working cycle of aselected cylinder of the internal combustion engine; determining acylinder-specific average value over the toothing times associated withthe working cycle of the selected cylinder; determiningcylinder-specific toothing time deviations of the toothing timesassociated with each working stroke of the selected cylinder from thedetermined cylinder-specific average value; determining acylinder-specific characteristic toothing time by determining thegeometric sum of the determined cylinder-specific toothing timedeviations; and determining the amount of energy released in the workingstroke of the selected cylinder of the internal combustion enginedepending on the determined cylinder-specific characteristic toothingtime, wherein the amount of energy released is indirectly proportionalto the determined cylinder-specific characteristic toothing time.

In a further embodiment, all toothing times occurring within a workingstroke of the selected cylinder are recorded and are associated with therelevant working stroke of the selected cylinder.

In a further embodiment, the cylinder-specific average value over thetoothing times associated with the working stroke of the selectedcylinder is determined based on toothing times that have been recordedduring a working stroke of a preceding working cycle of the internalcombustion engine.

In a further embodiment, any existing trend related to a variation ofthe toothing times, especially because of an increase or a reduction inthe revolution rate of the crankshaft of the internal combustion engine,is taken into account during the determination of the cylinder-specificaverage value.

Another embodiment provides a method for the regulation of the smoothrunning of an internal combustion engine with a plurality of cylinders,the method comprising determining, for each cylinder of the internalcombustion engine, the amount of energy released in the working strokeof said cylinder by a method as disclosed above, and adjusting at leastone combustion-relevant parameter, so that the amounts of energyreleased in the different cylinders are at least approximately the same.

In a further embodiment, the at least one combustion-relevant parameterrelates to a fuel supply path for the internal combustion engine.

Another embodiment provides a method for determining the cylinderpressure in different cylinders of an internal combustion engine with atleast two cylinders, wherein one cylinder is a lead cylinder fitted witha cylinder pressure sensor and the at least one other cylinder is anauxiliary cylinder, the method comprising determining, for each cylinderof the internal combustion engine, a relative value for the amount ofenergy released in the working stroke of said cylinder by a method asdisclosed above, measuring an absolute value for the cylinder pressurein the lead cylinder by means of the cylinder pressure sensor,determining a quantitative correlation between (a) the determinedrelative value for the amount of energy released in the working strokeof the lead cylinder and (b) the absolute value for the cylinderpressure in the lead cylinder, and calculating, for the at least oneauxiliary cylinder of the internal combustion engine, the absolute valueof the cylinder pressure in the at least one auxiliary cylinder based on(a) the determined quantitative correlation and (b) the determinedrelative value of the amount of energy released for the respective atleast one auxiliary cylinder.

Another embodiment provides a method for checking the plausibility of ameasurement signal of a cylinder pressure sensor of an internalcombustion engine that comprises at least two cylinders, each fittedwith a cylinder pressure sensor, the method comprising: determining, foreach of the at least two cylinders of the internal combustion engine, avalue for the amount of energy released in the working stroke of saidcylinder by a method as disclosed above; measuring, for each of the atleast two cylinders of the internal combustion engine, a value for thecylinder pressure in the respective cylinder by means of the respectivecylinder pressure sensor; and determining, for each of the at least twocylinders of the internal combustion engine, a respective quantitativecorrelation between (a) the determined value for the amount of energyreleased in the working stroke of the respective cylinder and (b) themeasured value for the cylinder pressure in the respective cylinder, andconsidering the at least two measured values for the respective cylinderpressure as correct measurement values if the at least two determinedquantitative correlations are equal within a specified tolerance.

In a further embodiment, the method further comprises: considering atleast one value for each cylinder pressure of the at least two measuredvalues for the respective cylinder pressure as an incorrect measurementvalue if the at least two determined quantitative correlations differfrom each other by more than the specified tolerance; and converting theat least one measurement value deemed to be incorrect into a modifiedmeasurement value for the cylinder pressure in each cylinder, so that amodified quantitative correlation between (i) the determined value forthe amount of energy released in the working stroke of the respectivecylinder and (ii) the modified measurement value is equal within thespecified tolerance to at least one quantitative correlation between (i)a determined value for the amount of energy released in the workingstroke of the respective cylinder and (ii) an associated measured valuefor the cylinder pressure in the respective cylinder, wherein the samerelates to at least one quantitative correlation for a cylinder fittedwith a cylinder pressure sensor whose measured values for the cylinderpressure are considered to be correct measurement values.

In a further embodiment, the method further comprises: operating theinternal combustion engine in a stable operating state, in which allcylinders make an at least approximately equal torque contribution tothe total torque of the internal combustion engine; measuring, in thestable operating state, for each of the at least two cylinders of theinternal combustion engine, a value for the cylinder pressure in therespective cylinder by means of the respective cylinder pressure sensor;comparing the values measured in the stable operating state with eachother; and if the values measured in the stable operating state deviatefrom each other by more than a further specified tolerance, adjusting asensor characteristic of at least one cylinder pressure sensor suchthat, taking into account the at least one adjusted sensorcharacteristic, the associated measurement values for the cylinderpressure in the different cylinders are equal at least within thefurther specified tolerance.

Another embodiment provides a device for determining the amount ofenergy released in the working stroke of a cylinder of an internalcombustion engine, the device comprising: a recording unit for recordinga time profile of the revolution rate of the crankshaft of the internalcombustion engine using toothing times, each representing a period oftime within which two adjacent teeth of a sensor disk, which isconnected to the crankshaft and which comprises an alternatingarrangement of teeth and tooth spaces along its circumference, pass areference position; and a data processing device for associating each ofthe toothing times with a respective working cycle of a selectedcylinder of the internal combustion engine, for determining acylinder-specific average value over the toothing times associated withthe working cycle of the selected cylinder, for determiningcylinder-specific toothing time deviations of each of the toothing timesassociated with the working stroke of the selected cylinder from thedetermined cylinder-specific average value, for determining acylinder-specific characteristic toothing time by determining thegeometric sum of the determined cylinder-specific toothing timedeviations, and for determining the amount of energy released in theworking stroke of the selected cylinder of the internal combustionengine depending on the determined cylinder-specific characteristictoothing time, wherein the amount of energy released is indirectlyproportional to the determined cylinder-specific characteristic toothingtime.

Another embodiment provides an engine controller for an internalcombustion engine of a motor vehicle, the engine controller comprising adevice as disclosed above for determining the amount of energy releasedin the working stroke of a cylinder of an internal combustion engine,wherein the engine controller is configured to carry out and/or tocontrol at least one of the following methods: a method as disclosedabove for determining the amount of energy released in the workingstroke of a cylinder of an internal combustion engine, a method asdisclosed above for regulating the smooth running of an internalcombustion engine with a plurality of cylinders, a method as disclosedabove for determining the cylinder pressure in different cylinders of aninternal combustion engine with at least two cylinders, wherein onecylinder is a lead cylinder fitted with a cylinder pressure sensor andthe at least one other cylinder is an auxiliary cylinder, and a methodas disclosed above for checking the plausibility of a measurement signalof a cylinder pressure sensor of an internal combustion engine thatcomprises at least two cylinders, each fitted with a cylinder pressuresensor.

Another embodiment provides a computer program for determining theamount of energy released in the working stroke of a cylinder of aninternal combustion engine, wherein the computer program is configuredto carry out any of the methods disclosed above when executed by aprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are discussed in detail below with reference to thedrawings, in which:

FIG. 1 shows a device for determining the amount of energy released inthe working stroke of a cylinder of an internal combustion engine.

FIG. 2 shows a diagram in which the time profile of high resolutionmeasured toothing times of a sensor disk coupled to a crankshaft of afour-cylinder four-stroke engine are plotted.

FIG. 3 shows an enlarged illustrated segment of the diagram shown inFIG. 2.

FIG. 4 shows how an absolute calculation of the individual cylinderpressures can be carried out for an internal combustion engine in whichonly one cylinder is fitted with a cylinder pressure sensor by formingrelationships of the amounts of energy released from the individualcylinders.

FIG. 5 shows a plausibility check of a plurality of measurement signalsof each cylinder pressure sensor using a comparison with an estimate ofa respective amount of energy released in a working stroke of a based onan analysis of measured toothing times.

FIG. 6 shows a possible procedure for a method of adjusting a sensorcharacteristic of a cylinder pressure sensor based on acylinder-selective comparison between (a) an estimated value for theamount of energy released in the working cycle of the cylinder involvedand (b) a measurement value for the cylinder pressure in the cylinderinvolved detected by a cylinder pressure sensor.

DETAILED DESCRIPTION

Embodiments of the invention determine the amount of energy released inthe working stroke of an internal combustion engine very accurately andwithout a complex sensor system.

Of course, features and details that are disclosed in connection withone of the methods described herein also apply here in connection withthe device, the engine controller and the computer program, and viceversa in each case, so that reference can always be alternatively madeto the individual aspects of the invention in relation to the disclosureof this invention.

Some embodiments provide a method for determining the amount of energyreleased in the working stroke of a cylinder of an internal combustionengine. The method comprises (a) recording a time profile of therevolution rate of the crankshaft of the internal combustion engineusing toothing times, each representing a period of time within whichtwo adjacent teeth of a sensor disk, which is connected to thecrankshaft and which comprises an alternating arrangement of teeth andtooth spaces along its circumference, pass a reference position, (b)associating the toothing times with a working stroke of a selectedcylinder of the internal combustion engine in each case, (c) determininga cylinder-specific average value over the toothing times associatedwith the working stroke of the selected cylinder, (d) determiningcylinder-specific toothing time deviations of each of the toothing timesassociated with the working stroke of the selected cylinder from thedetermined cylinder-specific average value, (e) determining acylinder-specific characteristic toothing time by determining thegeometric sum of the determined cylinder-specific toothing timedeviations and (f) determining the amount of energy released in theworking stroke of the selected cylinder of the internal combustionengine depending on the determined cylinder-specific characteristictoothing time, wherein the amount of energy released is indirectlyproportional to the determined cylinder-specific characteristic toothingtime.

The method described is based on the knowledge that the characteristictoothing time, which is given by the geometric (or Pythagorean) sum ofeach of the cylinder-specific toothing time deviations determined for aworking stroke of the selected cylinder, is a direct measure of theamount of energy released in a working stroke of the selected cylinder.

This means that the toothing time defined here is equivalent to therespective amount of energy released. Hence an absolute value for theamount of energy released in the working stroke of the respectivecylinder is not determined with the method described here, but only arelative value is determined. However, a proportionality factor betweensaid relative value and the respective absolute amount of energy is thesame for all cylinders, so that the relative values for differentcylinders can be set in relation to each other and thereby importantinformation about the operating state of the internal combustion engineand especially about the torque contributions of the individualcylinders can be obtained.

In comparison to known methods, the method described here has theadvantage that only a geometric sum has to be calculated and that thusno mathematically rather complex Fourier analysis of the toothing timesassociated with each working stroke of a selected cylinder of theinternal combustion engine has to be carried out. Furthermore, themethod described here is suitable both for gasoline engines and also fordiesel engines that are four-stroke internal combustion engines.

It is noted that the amount of energy released and thus also thecharacteristic toothing time is a direct measure or an equivalent of thetorque that is generated in the relevant working stroke of the selectedcylinder. In order to reduce any existing rough running of the internalcombustion engine, based on the amounts of energy released for thedifferent cylinders determined with the method described here, at leastan approximate balancing of the cylinders can be achieved by adjustingcombustion-relevant parameters cylinder-specifically for the individualcylinders, especially parameters describing the injection processes, sothat as a result each cylinder makes a contribution to the total torqueof the internal combustion engine that is as equal in magnitude aspossible.

The sensor disk used for carrying out the method described can comprisean edge structure in a known manner, which comprises an alternatingarrangement of a tooth and a tooth space in each case. A sensorassociated with the sensor disk, which detects the presence and theabsence of a tooth in the reference position, can produce a signal thatcan adopt at least two signal levels, wherein one of the signal levelsis associated with a tooth and the other is associated with a toothspace. The signal or the different signal levels can be produced in anyphysical manner. In particular, the signal can be an electrical signalthat is e.g. produced by a magnetic sensor (induction sensor),preferably a Hall sensor. However, other types of signal generation,e.g. optically by means of a light barrier, are also possible.

It is noted that in order to determine an absolute angular position ofthe crankshaft the sensor disk can comprise a reference marker that canbe detected by a suitable sensor system, e.g. the above-mentionedmagnetic sensor. In a known manner said marker can consist of e.g. twoteeth being omitted from the otherwise regular arrangement of analternating tooth and tooth space. Consequently, the sensor disk cane.g. comprise 60-2=58 teeth. In this connection it goes without sayingthat the toothing time that is associated with the omission of two teethis either corrected in a suitable manner or is no longer taken intoaccount for the subsequent process.

Expressed clearly, with the method described a revolution rate signal ofan internal combustion engine can be analyzed so that the energy contentor the amount of energy that is released in the working stroke of acylinder of the internal combustion engine can be estimated in amathematically particularly simple manner. A knowledge of said amount ofenergy can then be used to at least approximately achieve balancing ofthe individual cylinders of the internal combustion engine bycylinder-specific adjustment of injection parameters. The methoddescribed uses high resolution information about the toothing timessampled in a defined pattern over a specified observation time periodand stored in a buffer, especially of an engine controller.

According to another embodiment, all toothing times occurring within aworking stroke of the selected cylinder are recorded and associated withthe relevant working stroke of the selected cylinder. This has theadvantage that the time profile of the revolution rate of the crankshaftduring the respective working stroke of the selected cylinder isrecorded with the maximum possible accuracy. Hence the amount of energyreleased in the relevant working stroke of the selected cylinder canalso be determined with particularly high accuracy.

Preferably, the toothing times are recorded with high time resolution,e.g. in the region of at least a few μs (1 μs=10⁻⁶ seconds). Thesubsequent determinations or calculations of the cylinder-specificaverage value, the cylinder-specific toothing time deviations and thecylinder-specific characteristic toothing time preferably take placewith the same time resolution. The result of this is also that theaccuracy of the method described in relation to the determination of theamount of energy released is particularly high.

The term “working cycle” used below is to be understood in this documentto mean the entirety of the four strokes of a four-stroke engine, whichinclude as is well-known an induction stroke, a compression stroke, aworking stroke and an exhaust stroke. During a working cycle thecrankshaft of the internal combustion engine carries out tworevolutions.

In the case of a sensor disk with 60 teeth there is thus a total of 120transitions or toothing times between two adjacent teeth per workingcycle of the (four-stroke) internal combustion engine. In the case of afour-cylinder engine there are thus 30 toothing times associated witheach working stroke of a cylinder of the total of four cylinders.Consequently, optimal accuracy in the determination of the amount ofenergy released can be achieved if all 30 toothing times are taken intoaccount for the determination of the amount of energy released in therelevant working stoke of the selected cylinder with the methoddescribed.

In this connection it is noted that at least with modern internalcombustion engines accurate high time resolution information about theindividual toothing times is already maintained in an engine controller.The method described can thus be implemented without equipment changesby the suitable programming of an engine controller of an internalcombustion engine.

According to another embodiment, the cylinder-specific average valueover the toothing times associated with the working stroke of theselected cylinder is determined on the basis of toothing times that havebeen determined during a working stroke of a preceding working cycle ofthe internal combustion engine. This means that some of the mathematicalcalculations carried out with the method described here have alreadybeen completed in advance in a preceding working cycle of the internalcombustion engine. The requirement on a data processing device in whichthe mathematical calculations of the method described here are carriedout can therefore be reduced. The method described can consequently becarried out with an engine controller of only medium computing power.

Preferably, the cylinder-specific average value is determined over thosetoothing times that were recorded in the working cycle that immediatelyprecedes the current working cycle. In this way an unnecessarily longtime period is avoided between the recording of the toothing times thatare used exclusively for the cylinder-specific average value and therecording of those toothing times that are used (together with thetoothing times that are also exclusively used for the cylinder-specificaverage value) for the determination of the cylinder-specific toothingtime deviations. In this way a degradation of the accuracy as a resultof the use of toothing times that are associated with different workingcycles is reduced to a minimum.

According to another embodiment, during the determination of thecylinder-specific average value any prevailing trend relating to avariation of the toothing times, especially because of an increase or areduction of the revolution rate of the crankshaft of the internalcombustion engine, is taken into account. This has the advantage thateven in the case of a systematic variation of the toothing times, e.g.because of an acceleration or a deceleration of a motor vehicle drivenby the internal combustion engine, the cylinder-specific average valuecan already be determined in advance during a preceding working cyclewithout the fear of a degradation of the accuracy of the methoddescribed.

Other embodiments provide a method for the regulation of the smoothrunning of an internal combustion engine with a plurality of cylinders.Said method comprises (a) determining, for each cylinder of the internalcombustion engine, the amount of energy released in the working strokeof said cylinder by means of a method of the above-mentioned type fordetermining the amount of energy released in the working stroke of acylinder of an internal combustion engine, and (b) adjusting at leastone combustion-relevant parameter so that the amounts of energy releasedin the different cylinders are at least approximately equal.

The method described is based on the idea that the fuel combustion inthe individual cylinders can be adjusted based on a knowledge of theestimated cylinder-specific amounts of energy released in each workingstroke, which are a direct measure of the torque contributions of theindividual cylinders, so that all cylinders provide at leastapproximately the same torque contributions and thus maximum smoothrunning can be achieved.

Equalizing the individual torque contributions takes place by means ofan adjustment of parameters that are relevant to the fuel combustion ineach cylinder. Combustion-relevant parameters can relate to the airsupply path for the internal combustion engine or preferably to the fuelsupply path for the internal combustion engine.

A combustion-relevant parameter relating to the air supply path can e.g.be a charging pressure with which the air necessary for the combustionprocess is forced into the relevant cylinder of the internal combustionengine. Said charging pressure can be produced in a known manner, e.g.by a turbocharger. The combustion-relevant parameter can also be a rateof exhaust recycling that ensures in a known way that instead of pureair a mixture of air and exhaust gas from a previous combustion processof the cylinder involved is supplied. It is noted that the list of thecombustion-relevant parameters relating to the air supply path describedhere is not conclusive.

According to one embodiment, the at least one combustion-relevantparameter relates to a fuel supply path for the internal combustionengine.

The at least one combustion-relevant parameter relating to the fuelsupply path can e.g. be the start of a fuel injection, the injectionpressure of the fuel, the injected quantity of fuel, the number ofdiscrete injection processes (pre-injections) and/or the respectiveinjection amounts in the case of the use of at least one pre-injectionin addition to a main injection. The injection pressure can be measuredand/or adjusted in a known manner in a fuel supply system, e.g. in aso-called common rail system. It is however noted that the listdescribed here of combustion-relevant parameters is not conclusive.

The use of at least one of the combustion-relevant parameters describedhere relating to the fuel supply path has the advantage that the methoddescribed in this document can be implemented with conventional internalcombustion engines and conventional fuel supply systems without thisrequiring a hardware technology conversion of the internal combustionengine and/or the fuel supply system.

Other embodiments provide a method for determining the cylinder pressurein different cylinders of an internal combustion engine with at leasttwo cylinders, wherein one cylinder is a lead cylinder fitted with acylinder pressure sensor and the at least one other cylinder is anauxiliary cylinder. This method comprises (a) determining, for eachcylinder of the internal combustion engine, a relative value for theamount of energy released in the working stroke of said cylinder bymeans of a method of the above-mentioned type for determining the amountof energy released in the working stroke of a cylinder of an internalcombustion engine, (b) measurement of an absolute value for the cylinderpressure in the lead cylinder by means of the cylinder pressure sensor,(c) determining a quantitative correlation between (c1) the determinedrelative value of the amount of energy released in the working stroke ofthe lead cylinder and (c2) the absolute value of the cylinder pressurein the lead cylinder, and (d) calculating, for the at least oneauxiliary cylinder of the internal combustion engine, the absolute valueof the cylinder pressure in the at least one auxiliary cylinder based on(d1) the determined quantitative correlation and (d2) the relative valuedetermined for the amount of energy released for the at least onerespective auxiliary cylinder.

The method described for determining the cylinder pressure is based onthe idea that in a system with only one cylinder pressure sensorattached to a so-called lead cylinder the absolute cylinder pressures orcylinder pressure values in the auxiliary cylinders can be calculated by(a) forming a relationship between (a1) the absolute cylinder pressuremeasured with the cylinder pressure sensor and (a2) the with theabove-mentioned method for determining the amount of energy released inthe working cycle of a cylinder of an internal combustion engine and (b)by transferring the relationship formed to auxiliary cylinders, whichare not fitted with a cylinder pressure sensor, e.g. for cost reasons.

Based on a knowledge of the absolute cylinder pressure values (duringthe working strokes) of all cylinders, the absolute or relative torquecontributions of the individual cylinders can be determined in a knownway. As already explained above, the amounts of energy released in thedifferent cylinders can be adjusted by an adjustment of at least onecombustion-relevant parameter so that the torque contributions of theindividual cylinders to a total torque are at least approximately equal.

The internal combustion engine is then advantageously operated withmaximum smooth running or with minimum rough running.

With the method described, simply speaking a dimensionless measure ofthe relative amount of energy of at least two cylinders is initiallydetermined. Then for one of the two cylinders, which is designated asthe lead cylinder, the absolute cylinder pressure is measured by meansof a cylinder pressure sensor, which measures the pressure profile inthe lead cylinder and outputs a corresponding cylinder pressuremeasurement signal. Said cylinder pressure measurement signal representsthe torque contribution of the lead cylinder to the total torque of theinternal combustion engine. A subsequently determined relationship,which is also referred to as a quantitative correlation and which can bedefined by a simple proportionality factor, between the relative valueof the amount of energy released and the measured cylinder pressure isthen applied to the auxiliary cylinder. The absolute values of thecylinder pressure in the at least one auxiliary cylinder for this arecalculated based on (a) said relationship and (b) the respectivedetermined relative values for the respective released energy.

In this connection the term “cylinder pressure” can especially mean theso-called indicated mean pressure during the working stroke of therelevant cylinder. The term “cylinder pressure” can, however, also meana pressure profile as a function of time or as a function of acrankshaft angle, the pressure profile arising during the working strokeof the cylinder involved.

With the method described for determining the (absolute) cylinderpressure in different cylinders of an internal combustion engine, asimple cylinder pressure sensor system with only one cylinder pressuresensor can be used with an estimate of the relative individual torques,which as described above are correlated with the respective amount ofenergy released, and based on a single absolute cylinder pressuredetermination, to calculate the absolute values of the torquecontributions of all cylinders of the internal combustion engine. Fullcylinder pressure regulation can be achieved with only a single cylinderpressure sensor by suitable cylinder-specific adjustments ofcombustion-relevant parameters and thus accurate control of theindividual torque contributions can be achieved. For example, themanufacturing tolerances of fuel injectors can be compensated in thisway in a simple manner. Furthermore, by analyzing the cylinder-specificdifferences regarding the toothing time or the torque contributionderived therefrom, impermissibly large deviations can be diagnosed. Inthe case of excessive differences between the different cylinders, aconclusion may then be drawn regarding an incorrect operating state ofthe internal combustion engine.

Expressed simply, based on a knowledge of the individual torquecontributions, regulation of the individual torques can be carried outboth with respect to their differences from each other (targetedbalancing of the cylinders), which are to be avoided if possible, andalso with respect to their absolute values. The resulting controlinterventions such as e.g. an adjustment of the injection, can be usedfor a diagnosis of the operating state of the internal combustionengine. This can e.g. take place by monitoring limit values for therequired control interventions.

Furthermore, diagnostic functionality can advantageously be implementedwith the described method, by means of which impermissibly largedifferences between the individual cylinders, e.g. with respect to therespective (i) toothing times, (ii) torques and/or (iii) values derivedfrom the toothing times can be detected. If excessive differences aredetected, then a conclusion can be drawn regarding an incorrectoperating state of the internal combustion engine and e.g. a repairmeasure or maintenance work can be initiated.

Other embodiments provide a method for checking the plausibility of ameasurement signal from a cylinder pressure sensor of an internalcombustion engine comprising at least two cylinders, each cylinder beingfitted with a cylinder pressure sensor. This method comprises (a)determining, for each of the least two cylinders of the internalcombustion engine, a value for the amount of energy released in theworking stroke of said cylinder by means of the above-mentioned methodfor determining the amount of energy released in the working stroke of acylinder of an internal combustion engine, (b) measuring, for each ofthe at least two cylinders of the internal combustion engine, a valuefor the cylinder pressure in each of the cylinders by means of therespective cylinder pressure sensor, (c) determining, for each of the atleast two cylinders of the internal combustion engine, a quantitativecorrelation between (c1) the determined value for the amount of energyreleased in the working stroke of the respective cylinder and (c2) themeasured value for the cylinder pressure in the respective cylinder, and(d) regarding the at least two measured values for the respectivecylinder pressure as correct measurement values if the at least twodetermined quantitative correlations are equal within a specifiedtolerance.

The method described for checking the plausibility of a measurementsignal from a cylinder pressure sensor of an internal combustion engineis based on the idea that by a comparison of the quantitativecorrelations between (a) each estimated (relative) value for the amountof energy released in the working stroke of the respective cylinder and(b) each measured (relative or absolute) value for the cylinder pressurein the respective cylinder, it can be determined in a simple mannerwhether said quantitative correlations, which can each especially be asimple proportionality factor, are the same for all cylinders of theinternal combustion engine within a specified deviation that is alsoseen as tolerable. If this is the case, then it can be assumed therefromwith high reliability that the entire cylinder pressure sensor systemand especially all cylinder pressure sensors involved therein areworking properly. If in the event of a comparison of the differentdetermined quantitative correlations it should be revealed that at leastone correlation deviates too much from the other correlation(s), then itcan be assumed therefrom that at least one cylinder pressure sensor hasa certain defect.

As already explained above, the term “cylinder pressure” can mean theindicated mean pressure during the working cycle of the cylinderinvolved. The term “cylinder pressure” can, however, also be a pressureprofile as a function of time or as a function of a crankshaft angle,said pressure profile arising during the working stroke of the cylinderinvolved.

In some embodiments, the method further comprises (a) considering atleast one value for the respective cylinder pressure of the at least twomeasured values for the respective cylinder pressure as an incorrectmeasurement value if the at least two determined quantitativecorrelations deviate from each other by more than the specifiedtolerance, and (b) converting the at least one measurement value that isconsidered to be incorrect into a modified measurement value for thecylinder pressure in the respective cylinder, so that (b1) a modifiedquantitative correlation between (i) the determined value for the amountof energy released in the working stroke of the respective cylinder and(ii) the modified measurement value is equal within the specifiedtolerance to (b2) at least one quantitative correlation between (i) adetermined value for the amount of energy released in the working strokeof the respective cylinder and (ii) an associated measured value for thecylinder pressure in the respective cylinder, wherein the same relatesto at least one quantitative correlation for a cylinder that is fittedwith a cylinder pressure sensor, whose measured values for the cylinderpressure are considered to be correct measurement values. Thus in thecase of a defective cylinder pressure sensor system a correction can becarried out for that cylinder pressure sensor or for those cylinderpressure sensors by adjusting e.g. a sensor characteristic in a suitablemanner.

Expressed simply, the cylinder-specific torque contributions to a totaltorque of the internal combustion engine can be determined for aninternal combustion engine that is fitted with a cylinder pressuresensor system comprising a plurality of cylinder pressure sensors byusing the measured cylinder pressures. The different cylinders can bebalanced with respect to their respective torque contributions by acylinder-specific adjustment of combustion-relevant parameters. If,however, the cylinder pressure sensor system is defective, e.g. becauseof production errors and/or ageing effects, said defect can beidentified with high reliability with the method described here forchecking the plausibility of a measurement signal from a cylinderpressure sensor of an internal combustion engine, and may even becompensated by using a sensor characteristic modified in a suitablemanner.

According to one embodiment, the method further comprises (a) operatingthe internal combustion engine in a stable operating state in which allcylinders provide an at least approximately equal torque contribution tothe total torque of the internal combustion engine, (b) measuring, inthe stable operating state, for each of the at least two cylinders ofthe internal combustion engine, a value for the cylinder pressure in therespective cylinder by means of the respective cylinder pressure sensor,

-   -   (c) comparing the values measured in the stable operating state        with each other, and (d) if the values measured in the stable        operating state deviate from each other by more than another        specified tolerance, adjusting a sensor characteristic of at        least one cylinder pressure sensor such that by taking into        account the at least one adjusted sensor characteristic the        associated measurement values for the cylinder pressure in the        different cylinders are equal at least within the further        specified tolerance. This has the advantage that matching of the        individual sensor characteristics can be carried out during the        operation of the internal combustion engine. It is only        necessary that the described stable operating state of the        internal combustion engine exists at least for a relatively        short time period. As a result, defects in the cylinder pressure        measurement equipment can be diagnosed and compensated at the        same time by a suitable adjustment of at least one sensor        characteristic.

The stable operating state can e.g. be a deceleration phase of theinternal combustion engine, during which no fuel injection takes placein any of the cylinders. A deceleration phase is typically particularlycharacterized by maximum smooth running. This is because there is nofuel combustion in a deceleration phase. Therefore there can be nodifferences in the amounts of energy released in the differentcylinders. Therefore with correct sensor characteristics in such astable operating state with little rough running all basicallyoperational cylinder pressure sensors provide a similar measurementsignal.

The adjustment of the sensor characteristic can e.g. consist of a changeof a gradient or of a proportionality factor between the physical outputsignal of the cylinder pressure sensor involved and the respectivecylinder pressure measurement signal indicating the actual cylinderpressure.

Alternatively or in combination, the adjustment of the sensorcharacteristic can also include the use of a new offset-value.

Other embodiments provide a device for determining the amount of energyreleased in the working cycle of a cylinder of an internal combustionengine. The described device comprises (a) a recording unit forrecording a time profile of the revolution rate of the crankshaft of theinternal combustion engine using toothing times, each of which is aperiod of time within which two adjacent teeth of a sensor disk, whichis connected to the crankshaft and which comprises an alternatingarrangement of teeth and tooth spaces along its circumference, pass areference position, and (b) a data processing device (b1) forassociating each of the toothing times with a working stroke of aselected cylinder of the internal combustion engine, (b2) fordetermining a cylinder-specific average value over the toothing timesassociated with the working stroke of the selected cylinder, (b3) fordetermining cylinder-specific toothing time deviations of the respectivetoothing times associated with the working stroke of the selectedcylinder from the determined cylinder-specific average value, (b4) fordetermining a cylinder-specific characteristic toothing time bydetermining the geometric sum of the determined cylinder-specifictoothing time deviations and (b5) for determining the amount of energyreleased in the working stroke of the selected cylinder of the internalcombustion engine depending on the determined cylinder-specificcharacteristic toothing time, wherein the amount of energy released isindirectly proportional to the determined cylinder-specificcharacteristic toothing time.

The described device is also based on the idea that the characteristictoothing time, which is given by the geometric (or Pythagorean) sum ofthe respective cylinder-specific toothing time deviations determined fora working stroke of the selected cylinder, is a direct measure of theamount of energy released in a working stroke of the selected cylinderof the internal combustion engine. As already explained above, thecharacteristic toothing time defined here is a direct measure or anequivalent of the torque that is generated in the working strokeinvolved of the selected cylinder. Therefore, in order to reduce anyrough running of the internal combustion engine, balancing of thecylinders can be at least approximately achieved by adjustingcylinder-specific combustion-relevant parameters based on the amounts ofenergy released.

Other embodiments provide an engine controller for an internalcombustion engine of a motor vehicle. The described engine controllercomprises a device of the above-mentioned type for determining theamount of energy released in the working stroke of a cylinder of aninternal combustion engine. The engine controller described here issuitable for carrying out and/or for controlling at least one of theabove-mentioned methods (a) for determining the amount of energyreleased in the working stroke of a cylinder of an internal combustionengine, (b) for regulating the smooth running of an internal combustionengine with a plurality of cylinders, (c) for determining the cylinderpressure in different cylinders of an internal combustion engine with atleast two cylinders, wherein one cylinder is a lead cylinder fitted witha cylinder pressure sensor and the at least one other cylinder is anauxiliary cylinder, and (d) for checking the plausibility of ameasurement signal of a cylinder pressure sensor of an internalcombustion engine that comprises at least two cylinders, each of whichis fitted with a cylinder pressure sensor.

The engine controller described is based on the idea that theabove-described device can be implemented in an engine controller for aninternal combustion engine of a motor vehicle and that in this way, e.g.by means of suitable software, the amount of energy released in theworking stroke of a cylinder of an internal combustion engine can bedetermined in a simple manner.

In this connection it is noted that the engine controller described canalso work in conjunction with other components of the internalcombustion engine or of a motor vehicle in order to carry out someprocedural steps of the method described here. The engine controller canthus work in conjunction e.g. with an induction sensor to recordtoothing times and/or with at least one cylinder pressure sensor tomeasure the cylinder pressure in the cylinder involved.

Other embodiments provide a computer program for determining the amountof energy released in the working stroke of a cylinder of an internalcombustion engine. The computer program is configured to carry out theabove-mentioned method when executed by a processor.

For the purposes of this document, the naming of such a computer programis synonymous with the concept of a program element, of a computerprogram product and/or of a computer-readable medium, which containsinstructions for controlling a computer system for coordinating theoperation of a system or of a method in a suitable manner to achieve theeffects associated with the method according to the invention.

The computer program can be implemented as a computer-readableinstruction code in any suitable programming language such as e.g. inJAVA, C++ etc. The computer program can be stored on a computer-readablestorage medium (CD-ROM, DVD, Blueray disk, removable drive, volatile ornon-volatile memory, built-in memory or processor etc.). The instructioncode can program a computer or other programmable device, such asespecially a controller for an internal combustion engine of a motorvehicle, such that the desired functions are carried out. Furthermore,the computer program can be provided in a network such as e.g. theInternet, from which it can be downloaded when required by a user.

Embodiments of the invention can be implemented both by means of acomputer program, i.e., by means of software, and also by means of oneor more special electrical circuits, i.e. in hardware or even in anyhybrid form, i.e. by means of software components and hardwarecomponents.

Further advantages and features of the present invention are revealed inthe following exemplary description of currently preferred embodiments.

It is noted that the embodiments described below only represent alimited selection of possible embodiment versions of the invention. Inparticular, it is possible to combine the features of individualembodiments with each other in a suitable manner so that with theembodiment versions explicitly illustrated here a number of differentembodiments can be viewed as being publicly disclosed for the personskilled in the art.

FIG. 1 shows a device 100 for determining the amount of energy releasedin the working stroke of a cylinder 106 of an internal combustion engine108. The device 100 or some components of the device 100 can beimplemented in an engine controller for a motor vehicle. The device 100comprises a recording unit 102 for recording a time profile of therevolution rate of the crankshaft 110 of the internal combustion engine108 using toothing times, each of which represents a period of timewithin which two adjacent teeth of a sensor disk 105, which is connectedto the crankshaft 110 and which comprises and alternating arrangement ofteeth and tooth spaces along its circumference, pass a referenceposition. The device further comprises a data processing device 104. Thedata processing device 104 is configured or programmed (a) to associatethe toothing times with a respective working cycle of a selectedcylinder 106 of the internal combustion engine 108, (b) for determininga cylinder-specific average value over the toothing times associatedwith the working stroke of the selected cylinder 106, (c) to determinecylinder-specific toothing time deviations of each toothing timeassociated with the respective working stroke of the selected cylinder106 from the determined cylinder-specific average value, (d) fordetermining a cylinder-specific characteristic toothing time bydetermining the geometric sum of the determined cylinder-specifictoothing time deviations and (e) for determining the amount of energyreleased in the working stroke of the selected cylinder 106 of theinternal combustion engine 108 depending on the determinedcylinder-specific characteristic toothing time, wherein the amount ofenergy released is indirectly proportional to the determinedcylinder-specific characteristic toothing time.

FIG. 2 shows a diagram in which the time profile of high resolutionmeasured toothing times of a sensor disk coupled to a crankshaft of afour-cylinder four-stroke engine is plotted. The toothing time here isthe period of time within which two adjacent teeth of a sensor disk,which is coupled to the crankshaft of the internal combustion engineinvolved and which comprises an alternating arrangement of teeth andtooth spaces along its circumference, pass a reference position. Thetoothing time, which can be determined in a known manner, e.g. by meansof a magnetic sensor, consequently represents the current rate ofrotation or revolution rate of the crankshaft with high time resolution.A long toothing time corresponds to a low rate of rotation, a shorttoothing time corresponds to a high rate of rotation.

From the illustrated time profile of the toothing times an associationwith the working strokes of the individual cylinders of the internalcombustion engine can be carried out in a simple manner. Here the factcan be used that in the cylinder of the four-cylinder internalcombustion engine that is just in the working stroke the crankshaftaccelerates at the start of the working stroke starting from arelatively low rate of rotation. After a maximum rate of rotation hasbeen achieved within said working stroke, the rate of rotation decreasesslightly again before the next cylinder enters its working stroke and ina similar manner initially provides an increase of the rate of rotationtherein, and again provides a reduction of the rate of rotation afterexceeding a maximum.

Because the rate of rotation is indirectly proportional to the toothingtime, a characteristic shape results in the diagram of FIG. 2 for eachworking stroke of a cylinder, the shape being similar to the shape of aparabola that is open at the top and which is bounded in the horizontaldirection by two dashed vertical lines, one on the left side and one onthe right side.

The profile illustrated in FIG. 2 results in the case of a four-cylinderinternal combustion engine. For this reason the illustrated profile alsocomprises a periodicity of four such characteristic shapes. In FIG. 2each of said characteristic shapes is associated with a cylinder Z1, Z2,Z3 or Z4 of the four-cylinder internal combustion engine.

The horizontal dashed lines indicate the average value of the toothingtimes arising within the respective working stroke of the cylinderinvolved.

FIG. 3 shows a segment of the diagram shown in FIG. 2 in an enlargedillustration. In the case of the exemplary embodiment described here,the high resolution toothing time is sampled over a certain observationtime period of e.g. two working cycles of the internal combustion engineinvolved with a defined pattern and is stored in a buffer or a datamemory, especially a data memory of an engine controller. In the datamemory e.g. 30 toothing times per working cycle of the cylinder involvedcan be stored.

In contrast to previously known methods, the contents of the data memoryin the case of the method described here are not subjected to a complexfrequency analysis. Only the average value over the different toothingtimes is calculated and subtracted from each individual toothing time.Then the geometric sum is formed over the toothing times corrected inthis way by the average value. The next equation describes saidprocedure in a mathematical way:ZZ _(char)=√{square root over (Σ_(i=1) ^(N) ZZ _(i) ²)}

ZZ_(i) stands for the toothing time in the data memory at position icorrected by the average value. N is the number of toothing times orelements in the data memory. The data memory is a temporary toothingtime memory. The expression ZZ_(char) is the so-called characteristictoothing time, which represents an equivalent to the amount of energythat is released in the respective working stroke of the cylinderinvolved.

According to a version of the invention that is not explained in detailhere, prior to forming the geometric sum a constant component (e.g. fromthe last cycle or working cycle) or any toothing time trend (in the caseof a potentially present slight acceleration or deceleration of thecrankshaft) can also be removed.

The characteristic toothing time ZZ_(char) defined here represents anequivalent (indirect proportionality) of the torque contribution to thetotal torque that is provided by the cylinder involved with each workingstroke. By using an equality function with a suitable change of theinjection parameter (and thus of the respective torque contribution),the torque contributions of the individual cylinders can be brought intoagreement, so that as a result the smooth running of the internalcombustion engine is improved considerably.

In the case of the method described here, a complex FFT analysis can bedispensed with by removing the average value in the toothing timebuffering and forming the sum.

FIG. 4 shows how an absolute calculation of the individual cylinderpressures can be carried out in the case of an internal combustionengine in which only one cylinder is fitted with a cylinder pressuresensor by forming relationships of the amounts of energy released by theindividual cylinders. The upper diagram of FIG. 4 is identical to thediagram of FIG. 2. In the lower diagram of FIG. 4 the respectivecharacteristic toothing times are shown for each working stroke of oneof the cylinders of the internal combustion engine involved, which, asexplained above, represent an indirectly proportional equivalent to theamount of energy released in each working stroke.

Because only the average value over the toothing times involved has tobe formed prior to the calculation of a characteristic toothing time,the characteristic toothing times are shifted slightly to the right incomparison to the minima of the above-mentioned characteristic shapes atthe end of each working stroke.

In the case of the exemplary embodiment described here, the secondcylinder Z2 is the so-called lead cylinder. The other cylinders Z1, Z3and Z4 are so-called auxiliary cylinders. This means that only said leadcylinder is fitted with a cylinder pressure sensor, e.g. for costreasons. Therefore also only the torque contribution of said leadcylinder can be directly determined using the measurement data providedby the cylinder pressure sensor.

However, the individual amounts of energy that are released in theworking strokes of the different cylinders, and that are illustrated bycircles in FIG. 4, can be related to each other. The correspondingrelationships between the lead cylinder Z2 on the one hand and the othercylinders Z1, Z3 and Z4 on the other hand are illustrated in FIG. 4 bythe curved arrows that are shown in dashed form.

In order to indirectly determine the cylinder pressures in the cylindersZ1, Z3 and Z4, it is assumed that the amounts of energy released and therespective associated cylinder pressures for all cylinders are in thesame ratio to each other. Expressed simply, in a system with only onecylinder pressure sensor a calculation of the individual cylindertorques is carried out by forming the relationships of the amounts ofenergy released in the individual cylinders with each other (e.g. by aratio equation).

Based on said information, e.g. regulation of the relative cylindertorques in respect of minimizing the differences between the individualcylinder torques (balancing) can then be carried out. Moreover, theabsolute values of the cylinder pressures of the other cylinders canalso be regulated to specified target values based on the knowledge ofthe absolute cylinder pressure in the lead cylinder. The controlinterventions resulting therefrom can also be used for a diagnosis ofthe operation (state) of the internal combustion engine, e.g. byadjusting the parameters for fuel injection. This can e.g. be carriedout by monitoring limits for the control interventions for a definedtime period. Furthermore, diagnostic functionality can be provided thatcan detect differences of the individual cylinders (e.g. toothing times,torque or values derived from the toothing times) and can reliablyidentify impermissibly high deviations.

The method described here has the following advantages among others:

(A) The cylinder pressures in all cylinders can also be determined withhigh accuracy if only one (lead) cylinder is fitted with a cylinderpressure sensor. Because of this the complete provision of suitablecylinder pressure measurement equipment for each cylinder can beomitted.

(B) The regulation of cylinder pressure enables more accurate control ofthe torque contributions of the individual cylinders. Because of this,the manufacturing tolerances can be compensated in a simple andeffective manner, e.g. in the case of the fuel injectors used.

(C) Impermissibly high deviations can be diagnosed by suitable analysisof the cylinder differences with respect to the characteristic toothingtime or with respect to the torque contributions derived therefrom. Inthis way a repair of the internal combustion engine can be effected inmany cases at the correct time, i.e. before any further damage to theinternal combustion engine occurs.

(D) A variable describing the deviation of the individual cylinders fromeach other, such as especially the characteristic toothing time, can beused as an input for regulation that acts upon a suitable final controlelement (e.g. the injection). As a result, especially rough running ofthe internal combustion engine can be minimized.

It is noted that instead of the second cylinder Z2 of course any othercylinder can also be the lead cylinder fitted with the cylinder pressuresensor.

FIG. 5 shows a plausibility check of a plurality of measurement signalsof each cylinder pressure sensor using a comparison with an estimate ofa respective amount of energy released in a working stroke based on ananalysis of measured toothing times. In contrast to FIG. 4, in the lowerdiagram of FIG. 5 the dashed arrows are no longer present. Moreover, theassociated cylinder pressure that occurs during the respective workingstroke in the respective cylinder is plotted on an additional rightordinate. The cylinder pressures that have each been measured with acylinder-specific cylinder pressure sensor are each shown by a triangle.

As is apparent from FIG. 5, in the selected arbitrary scaling of the twoordinates for the cylinders Z1, Z2 and Z4 the triangles are each locatedat about half the height of the circles. Only in the case of cylinder Z3is the triangle noticeably higher than half the height of thecorresponding circle. On condition that the determination of thecharacteristic toothing times or the amounts of energy released is notincorrect, it can be assumed therefrom that the cylinder pressure sensorof cylinder Z3, or an analyzer connected downstream of said cylinderpressure sensor for the measurement signal of the cylinder pressuresensor, is defective. It may be that the error in the cylinder pressuremeasurement signal for the second cylinder can be compensated by anadjustment of a corresponding sensor characteristic.

FIG. 6 shows a possible procedure for a method for adjusting a sensorcharacteristic of a cylinder pressure sensor based on acylinder-selective comparison between (a) an estimated value for theamount of energy released in the working stroke of the cylinder involvedand (b) a measurement value for the cylinder pressure in the cylinderinvolved detected by a cylinder pressure sensor.

In the case of the method described here, a check is initially made in astep S2 as to whether a stable operating point or load point exists forthe internal combustion engine. Such a stable operating point or loadpoint e.g. exists if the fuel-injected amounts have small fluctuationsfrom working cycle to working cycle (the injected mass is slightlydynamic) and/or if the ambient conditions, such as e.g. the enginetemperature of the internal combustion engine, which can especially beindicated by the oil temperature, lies within determined limits. If astable operating point or load point does not exist, then the procedureis to wait until such a stable operating point or load point occurs at alater point in time. If no stable operating point or load point existsthen the characteristic toothing time or the amount of energy releasedis determined for the cylinder involved in a step S4 as explained above.Then the internal torque is determined for the cylinder involved in astep S6 by means of the cylinder pressure sensor mounted on thecylinder. In a subsequent step S8 a check is made as to whether (a) theresult of the estimation for the cylinder-specific torque contributionbased on the analysis of the toothing times involved and (b) themeasurement results for the torque contribution provided by the cylinderpressure measurement equipment are the same within predefined limits. Ifthis is the case, then there is no error. If not, then there is asuspected error, which can optionally be verified (not shown in FIG. 6)at another stable operating point or load point of the internalcombustion engine. According to the exemplary embodiment illustratedhere, in the case of an error a sensor characteristic can be adjusted ina step S10, e.g. by a change of a gradient or of a proportionalityfactor between the physical output signal of the cylinder pressuresensor involved and the respective cylinder pressure measurement signalindicating the actual cylinder pressure and/or by the use of a newoffset value. Using said adjusted sensor characteristic, the cylinderpressure sensor involved can then be used again for an engine controllerin the usual manner.

The method described here has the advantage that calibration of thecylinder pressure sensor system can be carried out during the operationof an internal combustion engine. It is only necessary that the internalcombustion engine is operated at least for a short time at a stableoperating point or load point. Moreover, with the method describeddefects in the cylinder pressure measurement equipment can be reliablydetected and potentially also diagnosed.

What is claimed is:
 1. A method for checking the plausibility of ameasurement signal of a cylinder pressure sensor of an internalcombustion engine that comprises at least two cylinders each fitted witha cylinder pressure sensor, the method comprising: recording a timeprofile of a revolution rate of a crankshaft of the internal combustionengine using toothing times, each toothing time representing a period oftime within which two adjacent teeth of a sensor disk pass a referenceposition, the sensor disk being connected to the crankshaft andcomprising an alternating arrangement of teeth and tooth spaces along acircumference of the sensor disk, associating the toothing times with arespective working cycle of a selected cylinder of the internalcombustion engine, determining a cylinder-specific average value overthe toothing times associated with the working cycle of the selectedcylinder, determining cylinder-specific toothing time deviations of thetoothing times associated with each working stroke of the selectedcylinder from the determined cylinder-specific average value,determining a cylinder-specific characteristic toothing time bydetermining the geometric sum of the determined cylinder-specifictoothing time deviations, determining an amount of energy released inthe working stroke of the selected cylinder of the internal combustionengine depending on the determined cylinder-specific characteristictoothing time, wherein the amount of energy released is indirectlyproportional to the determined cylinder-specific characteristic toothingtime, measuring, for each of the at least two cylinders of the internalcombustion engine, a value for the cylinder pressure in the respectivecylinder using the respective cylinder pressure sensor, determining, foreach of the at least two cylinders of the internal combustion engine, arespective quantitative correlation between: (a) the determined valuefor the amount of energy released in the working stroke of therespective cylinder, and (b) the measured value for the cylinderpressure in the respective cylinder, and using the at least two measuredvalues for the respective cylinder pressure as correct measurementvalues if the at least two determined quantitative correlations areequal within a specified tolerance, operating the internal combustionengine in a stable operating state, in which all cylinders make an atleast approximately equal torque contribution to the total torque of theinternal combustion engine, measuring, in the stable operating state,for each of the at least two cylinders of the internal combustionengine, a value for the cylinder pressure in the respective cylinderusing the respective cylinder pressure sensor, comparing the valuesmeasured in the stable operating state with each other, and if thevalues measured in the stable operating state deviate from each other bymore than a further specified tolerance, adjusting a sensorcharacteristic of at least one cylinder pressure sensor such that,taking into account the at least one adjusted sensor characteristic, theassociated measurement values for the cylinder pressure in the differentcylinders are equal at least within the further specified tolerance. 2.The method of claim 1, further comprising: considering at least onevalue for each cylinder pressure of the at least two measured values forthe respective cylinder pressure as an incorrect measurement value ifthe at least two determined quantitative correlations differ from eachother by more than the specified tolerance, and converting the at leastone measurement value deemed to be incorrect into a modified measurementvalue for the cylinder pressure in each cylinder, so that a modifiedquantitative correlation between (i) the determined value for the amountof energy released in the working stroke of the respective cylinder, and(ii) the modified measurement value is equal within the specifiedtolerance to at least one quantitative correlation between (i) adetermined value for the amount of energy released in the working strokeof the respective cylinder, and (ii) an associated measured value forthe cylinder pressure in the respective cylinder, wherein the samerelates to at least one quantitative correlation for a cylinder fittedwith a cylinder pressure sensor whose measured values for the cylinderpressure are considered to be correct measurement values.